Seed Oils and Weight Loss: What Studies Show

If you've heard that seed oils are toxic or inflammatory, you've probably looked at the canola in your cupboard with some suspicion. Some people are swapping it for butter or beef tallow. Clients ask me about it most weeks.

The human research has been done, and it's less dramatic than the videos.

The theory

Soybean, corn, grapeseed, and regular sunflower and safflower oil are high in linoleic acid, a type of omega-6 fat. The worry is a chain reaction. Linoleic acid gets turned into arachidonic acid, another fat, and arachidonic acid is a building block for signals your body uses to turn inflammation up.

In people, the chain stalls at step two. A review of human studies found that eating up to six times more linoleic acid, or up to 90 percent less, left arachidonic acid in the blood about where it was. The body appears to regulate that step on its own.

What the controlled trials found

A controlled trial assigns people at random to eat more linoleic acid or not, then measures inflammation in their blood. CRP, C-reactive protein, is the standard test. IL-6 and TNF-alpha are signals the immune system uses to turn inflammation up.

A 2012 systematic review found 15 such trials in healthy people. None found a clear rise in CRP or the other markers they measured. A 2017 meta-analysis, which means many trials pooled into one result, covered 30 trials and 1,377 people. CRP, IL-6 and TNF-alpha stayed about the same when linoleic acid went up. The authors noted one caveat, which is that in the trials where linoleic acid intake increased the most, CRP might rise.

These trials test the fear directly, and the markers held steady. That's the claim the evidence supports, and it's a narrow one. More seed oil didn't mean more inflammation. Whether seed oils are good for you is a different question, and these trials didn't ask it.

The long-term picture

Observational studies measure people as they are and look for patterns. They can show a link and can't prove cause. With that said, one analysis pooled 30 of them across 13 countries, 68,659 people followed for up to 32 years, with linoleic acid measured in their blood or body fat rather than guessed from food diaries. People with higher levels had a 22 percent lower rate of death from cardiovascular disease, meaning the heart and blood vessels, a 12 percent lower rate of ischemic stroke, the kind caused by a blocked vessel, and a 7 percent lower rate of cardiovascular disease overall.

Critics point to the Minnesota Coronary Experiment from the late 1960s, which swapped saturated fat for corn oil and saw cholesterol fall without deaths falling. It's a fair point about that one trial, and it never measured inflammation. A link across 30 studies in 13 countries is hard to wave off.

Olive oil has the strongest case

Extra-virgin olive oil is mostly monounsaturated fat, a different family from the omega-6 in seed oils, and it carries polyphenols, plant compounds that act as antioxidants.

A meta-analysis of 30 trials with 3,106 people found that olive oil lowered CRP by 0.64 mg/L and lowered IL-6 as well. Most of those trials gave olive oil as part of a Mediterranean-style diet, and that's where the evidence is strongest. Extra-virgin olive oil on its own, pulled out of the diet pattern, shows a weaker signal. Make it your everyday oil and let the rest of the plate help.

Avocado oil hasn't been studied much

Avocado oil is also mostly monounsaturated. The problems are how little it has been studied in people and what's in the bottle. The human inflammation evidence comes down to one small test of a single meal in 13 adults, so it tells us nothing about eating it for years.

The bottle problem is worse. In a 2020 analysis by UC Davis researchers, 82 percent of 22 avocado oil samples sold in the US were either rancid before their expiration date or mixed with other oils, and three bottles labeled pure or extra virgin were nearly all soybean oil.If you buy avocado oil, buy a brand that publishes testing. If that sounds like homework, buy olive oil.

Heat is where the bad reputation comes from

Heat breaks oil down, and labs measure the damage as polar compounds. Countries that regulate frying oil set the throw-out point around 25 percent. In an independent lab test that fried food nine times in each oil, high-oleic sunflower oil, a version bred to be mostly monounsaturated like olive oil, held up best. Virgin avocado and virgin olive oil stayed under 20 percent. Regular sunflower oil crossed the limit, and refined avocado oil came close to it.

Fat type, processing and the oil's natural antioxidants predicted the result far better than the word seed on the label. Nine rounds of frying is restaurant territory. A home sauté is one short round, and it doesn't get an oil anywhere near those numbers.

Butter and tallow aren't an upgrade

Linoleic acid is an essential fatty acid, which means your body can't make it and it has to come from food. The 2025 to 2030 Dietary Guidelines say to "prioritize oils with essential fatty acids, such as olive oil. Other options can include butter or beef tallow." Seed oils aren't named.

The outcome data lean one way. In three large cohorts totaling 221,054 adults followed for up to 33 years, the people eating the most butter had a 15 percent higher risk of death than those eating the least, and the people eating the most plant oil had a 16 percent lower risk. A statistical model estimated that swapping 10 grams of butter a day, about two teaspoons, for plant oil was linked to a 17 percent lower risk. Canola, soybean and olive oil were each linked to lower death rates on their own. This is observational, so it shows a link, which is weaker than proof. It's still the opposite of what the swap-to-butter advice predicts.

What this has to do with weight loss

Every cooking oil is about 120 calories per tablespoon, whichever bottle it comes from, and butter is close to 100. For a client trying to lose weight, the oil question is mostly a calorie question, and the answer is to measure it. Two tablespoons poured freehand into a pan is often three, and that's 360 calories before the food goes in.

The other weight loss link is where seed oils live in the average diet, which is fried food and packaged snacks. Cut back on those and most of the seed oil goes with them, along with a lot of calories that weren't filling you up. That one isn't from a study. It's what I see with clients.

What to cook with

For everyday cooking and dressings, use extra-virgin olive oil. For high-heat searing, use high-oleic sunflower oil or a virgin avocado oil from a brand that tests its product. If canola or soybean oil is what you have at home, it's fine, and stressing over it isn't worth your attention.

Cut the fried food and the packaged snacks first. Then pick a bottle from that list and stop thinking about it.

Sources

1. Rett & Whelan, Nutrition & Metabolism, 2011. Increasing dietary linoleic acid does not increase tissue arachidonic acid content in adults consuming Western-type diets: a systematic review. Eating up to six times more linoleic acid, or up to 90 percent less, left blood arachidonic acid about where it was.
2. Johnson & Fritsche, Journal of the Academy of Nutrition and Dietetics, 2012. Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic review of randomized controlled trials. 15 trials in healthy people: none found a clear rise in CRP or the other blood markers of inflammation measured.
3. Su et al., Food & Function, 2017. Dietary linoleic acid intake and blood inflammatory markers: a systematic review and meta-analysis of randomized controlled trials. 30 trials, 1,377 people: CRP, IL-6 and TNF-alpha stayed about the same; CRP might rise where linoleic acid went up the most.
4. Marklund et al., Circulation, 2019. Biomarkers of dietary omega-6 fatty acids and incident cardiovascular disease and mortality. 30 cohorts, 68,659 people: higher blood linoleic acid was linked to 22 percent lower cardiovascular death and 12 percent lower ischemic stroke. Observational.
5. Schwingshackl, Christoph & Hoffmann, Nutrients, 2015. Effects of olive oil on markers of inflammation and endothelial function: a systematic review and meta-analysis. 30 trials, 3,106 people: olive oil lowered CRP by 0.64 mg/L and lowered IL-6.
6. Zhang et al., JAMA Internal Medicine, 2025. Butter and plant-based oils intake and mortality. 221,054 adults followed up to 33 years: highest butter intake linked to 15 percent higher death risk; highest plant oil intake linked to 16 percent lower. Observational.
7. Ramsden et al., BMJ, 2016. Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73). Swapping saturated fat for corn oil lowered cholesterol by about 14 percent without lowering deaths. The trial never measured inflammation.
8. Green & Wang, Food Control, 2020. Quality and purity evaluation of avocado oil sold in the US (UC Davis). Of 22 avocado oil samples sold in the US, at least 82 percent were rancid before their expiration date or mixed with other oils.
9. US Department of Health and Human Services and US Department of Agriculture, 2026. Dietary Guidelines for Americans, 2025-2030, fats and oils guidance. Says to prioritize oils with essential fatty acids, such as olive oil, and that other options can include butter or beef tallow. Seed oils aren't named.
10. Holgado et al., Grasas y Aceites, 2024. Performance of virgin and refined avocado oils during deep-frying and thermoxidation simulating frying in comparison with olive and sunflower oils. Nine frying rounds per oil, measured as polar compounds: high-oleic sunflower held up best, virgin avocado and olive stayed under 20 percent, regular sunflower crossed the 25 percent limit.